code: def test_empty_graph():
      graph = TopologyGraph({})
      assert not graph.origin_nodes
  
imports: import asyncio
  import copy
  from collections import defaultdict
  from typing import List
  import pytest
  from docarray import Document, DocumentArray
  from jina.serve.runtimes.gateway.graph.topology_graph import TopologyGraph
  from jina.types.request import Request
  from jina.types.request.data import DataRequest
label_desc: Use image node_id 16 for calling a global function with example usage: test_empty_graph() without return types